Faculty of Science and Technology
MASTER’S THESIS
Study program/Specialization:
Petroleum Geosciences Engineering
Spring, 2019 Open Writer:
Cicilie Trede
(Writer’s signature) Faculty supervisor: Dora Luz Marin Restrepo
Title of thesis:
3D seismic analysis of the Basement to Early Cretaceous in the Selje High, Slørebotn Sub-basin and Måløy Slope, southern Norwegian Sea.
Credits (ECTS): 30 Keywords:
Agat Formation Selje High
Slørebotn Sub-basin Måløy Slope
Jurassic – Cretaceous rifting Source area
Spectral decomposition
Pages: 126
Stavanger, 15th June, 2019
Copyright by Cicilie Trede
2019
3D seismic analysis of the Basement to Early Cretaceous in the Selje High, Slørebotn Sub-basin and Måløy Slope, southern Norwegian Sea.
by Cicilie Trede
Msc Thesis
Presented to the Faculty of Science and Technology The University of Stavanger
Norway
The University of Stavanger 2019
I
Acknowledgements
Firstly, I would like to thank my supervisor Dora Marin for her continuous support and motivation throughout this project.
I would also like to thank Wiktor Weibull and Alejandro Escalona for valuable feedback on this thesis. Additionally, I am grateful to Jennifer Cunningham for the technical support with
the GeoTeric software.
Thanks to my family and friends for the continued encouragement and support throughout my Master’s degree.
Finally, I would like to thank my helpful classmates.
II Abstract
3D seismic analysis of the Basement to Early Cretaceous in the Selje High, Slørebotn Sub-basin and Måløy Slope, southern Norwegian Sea.
Cicilie Trede
The University of Stavanger, 2019
Supervisors: Dora Luz Marin Restrepo
Several coarse-grained sedimentary rocks have been reported in the Upper Jurassic and the Cretaceous succession in the southern Norwegian Sea and the northern North Sea. The Jurassic interval has been interpreted as syn-rift deposits while the Cretaceous succession is considered post-rift strata. However, the timing of the fault activity in the Early Cretaceous has been debated. In addition, the coarse-grained sandstones of the Early Cretaceous Agat Formation are still poorly understood, and few studies are related to this interval in the Selje High area.
Furthermore, the basement in the northern North Sea in the Utsira High has proven to bear potential as a reservoir in fractured basement rocks, nevertheless, further studies are needed on this in the northern North Sea. This study aims to improve the understanding of the fault activity in the Late Jurassic to Early Cretaceous and the lateral distribution of the coarse-grained formations of Early Cretaceous, thus improve the paleogeographic understanding in the area surrounding Selje High. A further purpose is to improve the understanding of the Pre-Devonian basement.
This is accomplished by using a dataset containing 3D and 2D reflection seismic and six wells.
In addition to traditional seismic interpretation, spectral decomposition was used to highlight the Basement and the Agat Formation.
Main findings comprise of three fault families of Late Jurassic age exposed to rotation which uplifted basement highs. These were later reactivated during the late Early Cretaceous.
However, a second theory is suggested favoring thermal subsidence in the Early Cretaceous.
The main controlling factor on the lateral extent of the coarse-grained Agat Formation is the rifting bathymetry from the Late Jurassic and eustatic sea level changes. The Agat Formation
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is sourced as slumps from the Selje High and submarine fans from the eastern platform area through submarine canyons of Late Jurassic age. These submarine canyons are found in the basement of structural highs and are present up until the Base Cretaceous Unconformity.
Lastly, the basement proved to have minor faulting or fractures in the Selje High, thus possibly bearing the potential as a reservoir.
IV
TABLE OF CONTENT
INTRODUCTION ... 1
1.1 Objectives ... 2
1.2 Previous work ... 4
Seismic expression of active rift basins ... 4
1.2.2 Rifting events in the Late Jurassic and Early Cretaceous and its influence on sedimentation ... 5
1.2.3 Depositional model of the Agat Formation ... 6
GEOLOGICAL SETTING ... 8
2.1 Tectonic setting ... 10
2.1.1 Jurassic ... 10
2.1.2 Cretaceous ... 11
2.2 Stratigraphy ... 13
2.2.1 Pre-Devonian ... 15
2.2.2 Pre-Jurassic ... 16
2.2.3 Jurassic ... 16
2.2.4 Cretaceous ... 16
2.2.5 Post-Cretaceous ... 19
DATA AND METHODOLOGY ... 20
3.1 Data ... 20
3.2 Methodology ... 22
3.2.1 Well correlation ... 22
3.2.2 Seismic interpretation ... 22
3.2.3 Spectral decomposition ... 25
OBSERVATIONS AND RESULTS ... 27
4.1 Structural well correlation ... 27
4.2 Seismic interpretation of the main intervals ... 31
4.2.1 Basement ... 31
4.2.2 Pre-BCU ... 56
4.2.3 Agat Formation ... 70
DISCUSSION ... 88
5.1 Tectonic evolution ... 88
5.1.1 Late Jurassic ... 88
5.1.2 Early Cretaceous ... 91
5.2 Basement structure and implications for exploration ... 97
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5.3 Paleogeography ... 99
5.3.1 Late Jurassic ... 99
5.3.2 Early Cretaceous Agat Formation ... 101
CONCLUSIONS ... 105
6.1 Future work ... 106
VI
TABLE OF FIGURES
Figure 1: Study area (red square) and the structural elements of the northern North Sea and the southern Norwegian Sea. Gn H = Gnausen High, Go H = Gossa High, Gi H = Giske High, M H = Makrell Horst, and S H = Selje High. Modified from (Blystad et al., 1995). ... 3 Figure 2: An idealized section of an active rift basin with a description of seismic
characteristics for the four tectonic systems tracts. 1) Rift initiation, 2) Rift climax, 3)
Immediate post-rift, and 4) Late post-rift. (Prosser, 1993) ... 4 Figure 3: Idealized section to illustrate Cretaceous infilling of half-grabens typical for the Cretaceous sedimentary succession in the Norwegian Sea. The Cretaceous infill has been interpreted as the post-rift infilling of hanging wall sub-basins following Jurassic faulting and extensions (Færseth & Lien, 2002). ... 5 Figure 4: Regional seismic line crossing from West to East, from the Slørebotn Sub-basin through the Selje High to the Måløy Slope and the platform area. A) Uninterpreted seismic line. B) Interpreted seismic line. The interpretation in the west close to the platform area is uncertain. ... 9 Figure 5: Regional line showing from the NW to the SSE from the Slørebotn sub-basin, Måløy Terrace and Selje High. This regional line is along the same line as interpreted in Figure 4. Modified by (Brekke, 2000). ... 12 Figure 6: Lithostratigraphic chart of the Northern North Sea from the Tampen Spur and Horda Platform. The lithostratigraphy in the study area is believed to be similar. Modified from (NPD, 2014). ... 14 Figure 7: Cores from well 6204/10-2R showing the Åsgard Formation from 1951m to
1961m. The cores show dark fine-grained sediments. Collected from NPD fact pages (NPD, 2019). ... 18 Figure 8: Chronostratigraphic chart for the Cretaceous from the Sogn Graben, Måløy
Terrace, and Selje High. (Modified from (Vergara, Brunstad, Nordlie, Chranock, & Gradstein, 2006) ... 19 Figure 9: An overview of the dataset from the study area. Blue lines are 2D seismic lines, while the black box show the seismic 3D cube. Black circles are the wells. The Norwegian mainland are located to the east. ... 21
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Figure 10: A close-up of the seabed highlighting the polarity of the seismic cube. ... 22 Figure 11: Seismic displayed with the synthetic seismogram to highlight the good correlation between the synthetic and the seismic. ... 22 Figure 12: Seismic well-tie for well 6204/10-1. The following well logs displayed are: GR, sonic, density, acoustic impedance, and reflection coefficient. The top of present formations are marked. ... 23 Figure 13: Seismic well-tie for well 6204/11-2. The following well logs displayed are: GR, sonic, density, acoustic impedance, and reflection coefficient. The top of present formations are marked. ... 23 Figure 14: Structural well correlation between all wells in the study area going from
southwest to northeast (yellow line in map), through the Selje High, Måløy Slope, and
Slørebotn Sub-basin. The correlation is from Basement until the top of Upper Cretaceous. .. 30 Figure 15: Structural map of the basement with all the faults divided into three fault families.
Fault family 1(FF1) in yellow, Fault family 2 (FF2) in green and Fault family 3 (FF3) in blue.
... 32 Figure 16: Seismic line displaying faults from fault family 1. Uninterpreted seismic line. ... 34 Figure 17: Seismic line displaying faults from fault family 1. Interpreted seismic line
showing the Selje High and the main characteristics of fault family 1. Red arrows indicates lap relationships. Two major faults bounding the ... 35 Figure 18: Seismic line displaying faults from fault family 2. Uninterpreted seismic line. ... 37 Figure 19: Seismic line displaying faults from fault family 2. Interpreted seismic line
showing the Selje High and the main characteristics of fault family 2. ... 38 Figure 20: Seismic line displaying faults from fault family 3. Uninterpreted seismic line. ... 40 Figure 21: Seismic line displaying faults from fault family 3. Interpreted seismic line
showing the faults in the north and the main characteristics of fault family 3. ... 41 Figure 22: Basement map. a) Surface map of the basement in time (ms). b) Structural map of the basement in time (ms) with faults active during the period. Contour interval: 300ms.
Elevated basement areas are marked as the Selje High and the Platform area. ... 43
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Figure 23: Seismic inline crossing along the platform area to highlight the basement incisions observed in the structural map. The seismic line is along the yellow line in the structural basement map. Uninterpreted section. ... 45 Figure 24: Seismic inline crossing along the platform area to highlight the basement incisions observed in the structural map. The seismic line is along the yellow line in the structural basement map. The top basement (blue) and BCU (red) are marked in the seismic line, along with onlap within the submarine canyons. Folding is also observed. ... 46 Figure 25: Basement horizon seen from above with constant bandwidth frequency
decomposition in GeoTeric. The result of a frequency decomposition with an RGB color blend (17Hz - 24Hz - 31Hz) displayed on the basement surface. Submarine canyons/gullies, incisions and the elevated basement areas are marked. ... 50 Figure 26: Basement horizon seen from above with constant high definition frequency decomposition in GeoTeric. The result of a frequency decomposition using with an RGB color blend (15Hz - 30Hz - 45Hz) displayed on the basement surface. Submarine
canyons/gullies, incisions and the elevated basement areas are marked. ... 51 Figure 27: Basement horizon with constant bandwidth frequency decomposition in GeoTeric.
The result of a frequency decomposition with an RGB color blend (17Hz - 24Hz - 31Hz) displayed on the basement surface and seen from the NW to highlight the submarine canyons along the fault plane of the platform area. ... 52 Figure 28: Basement horizon with HD frequency decomposition in GeoTeric. The result of a frequency decomposition with an RGB color blend (15Hz - 30Hz - 45Hz) displayed on the basement surface and seen from the NW to highlight the submarine canyons along the fault plane of the platform area. ... 53 Figure 29: Close-up of the highest part of the Selje High, marking the possible slump scarps in the basement with a stippled red line. ... 54 Figure 30: Close-up of the top of Selje High seen from above, highlighting the possible faults or fractures oriented N-S. ... 55 Figure 31: Maps of the Base Cretaceous Unconformity (BCU). A) Surface map of the BCU in time (ms) B) Structural map of the BCU in time (ms) with faults active during the period.
Contour interval: 200ms. Elevated basement areas are marked as the Selje High and the Platform area. ... 59
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Figure 32: Time thickness map between the BCU and the Basement with thickness in time (ms). The elevated basement areas have lowest thickness (red) and the structural lows have varying thickness of 100-400 ms. ... 60 Figure 33: Seismic XL 300 highlighting the Pre-BCU wedges and the Agat Formation.
Onlaps are marked for the Agat Formation. Seismic line going from A to A’ as marked in the structural map. ... 63 Figure 34: Seismic XL 404 highlighting the Pre-BCU wedges and the Agat Formation.
Downlap is observed of the Agat Formation on the BCU. Seismic line going from B to B’ as marked in the structural map. ... 64 Figure 35: Seismic XL 1073 highlighting the Pre-BCU wedges and the Agat Formation. Two wedges are defined in this seismic line. Onlap in the hanging wall is observed for the Agat Formation in both wedges. Seismic line going from C to C’ as marked in the structural map.
... 65 Figure 36: Seismic XL 1125 highlighting the Pre-BCU wedges and the Agat Formation. The same two wedges as observed in Figure 36 have now merged to one large wedge with a minor fault in between. Onlap in the hanging wall is observed for the Agat Formation on both sides of the minor fault. Seismic line going from D to D’ as marked in the structural map ... 66 Figure 37: Seismic XL 1550 highlighting the Pre-BCU wedges and the Agat Formation. The entire wedge is not observed in seismic, however, the Pre-BCU succession is likely pinching out in the east. A Pre-BCU wedge is also observed on the west of Selje High thickening towards the Sogn-Graben. Seismic line going from E to E’ as marked in the structural map . 67 Figure 38: A map with the structural elements highlighting the location of the Pre-Cretaceous wedges in the area. As explained above. These are located in relation to the faults... 68 Figure 39: Structural maps of the Agat Formation. a) Top Agat b) Base Agat Maps of the Base Cretaceous Unconformity (BCU). a) Surface map of the BCU in time (ms) b) Structural map of the BCU in time (ms) with faults active during the period. Contour interval: 200ms.
Elevated basement areas are marked as the Selje High and the Platform area. ... 73 Figure 40: Time thickness map for the Agat Formation A) Time thickness map between the BCU and Top Agat Formation and B) Time thickness map between the Base Agat Formation and the Top Agat Formation. The faults are displayed in the figure to highlight that these are marking the lateral extent of the Agat Formation. ... 74
X
Figure 41: Seismic X-line 404 highlighting the Agat Formation with a yellow stippled line.
The Agat Formation is downlapping the BCU in the west marked with red arrows. ... 77 Figure 42: Seismic X-line 1125 highlighting the Agat Formation with a yellow stippled line.
The Agat Formation is onlapping the BCU in the west marked with red arrows. A mound or a fold is observed in the hanging wall of Selje High. ... 78 Figure 43: Uninterpreted random seismic line between well 6204/11-1 and 6204/11-2. ... 81 Figure 44: Interpreted random seismic line between well 6204/11-1 and 6204/11-2. Agat Formation is onlapping the BCU and a GWC are defined in the anticlinal shape of well 6204/11-1. ... 82 Figure 45: Top Agat Formation with a constant bandwidth spectral decomposition with an RGB color blend (17Hz – 30Hz – 42Hz). Circular and linear geometries are observed in the south and in the north, respectively. ... 84 Figure 46: Iso-proportional slicing between top and base of Agat Formation. Constant
Bandwidth Spectral decomposition with a RGB color blend (17Hz – 30Hz – 42Hz). a) Top Agat b) Upper Bound c) Bound 1. The same geometries as observed in Figure 46 are
observed here. Marked in a). ... 85 Figure 47: Iso-proportional slicing between Top and Base of Agat Formation. Constant Bandwidth Spectral decomposition with a RGB color blend (17Hz – 30Hz – 42Hz). d) Bound 2, e) Lower Bound, f) Base Agat. The same geometries as observed in Figure 46 are less evident in these three slices, where the color has dimmed significantly. ... 86 Figure 48: Structural model of the Late Jurassic highlighting the fault activity and rotation of the Selje High. The Jurassic or older interval are dominated by deep marine shales. Erosion in the foot wall is marked along with possible gravity flows in the hanging wall. The Jurassic or older onlaps the Basement in the hanging wall. ... 90 Figure 49: Structural model of the Early Cretaceous theory 1 highlighting the tectonic
quiescence period around Selje High. Erosion in the foot wall is marked along with possible gravity flows in the hanging wall. The Agat Formation onlaps the Basement, while the Åsgard Formation is truncated. Åsgard Formation partially deposited on the Selje High. ... 93 Figure 50: Structural model of the Early Cretaceous alternative theory highlighting the reactivation of the fault activity around Selje High. Erosion in the foot wall is marked along with possible gravity flows in the hanging wall. The Agat Formation onlaps the Basement,
XI
while the Åsgard Formation is truncated. Åsgard Formation partially deposited on the Selje High. Wedge geometries in the Åsgard and Agat Formation are observed. ... 96 Figure 51: Submarine canyons observed downslope of the Equatorial Guinean seafloor. The figure displays to different canyons types. Type 1 favors submarine fans, while type 2 are more mud-rich and less carved into the slope (Jobe, Lowe, & Uchytil, 2011). ... 98 Figure 52: Conceptual sketch of the Late Jurassic. The Selje High is present during this period, and all the major faults in the study area are present in the Late Jurassic. Some sandy gravity flows are drawn in the northern area to imply that Sognefjord and Heather Sandstone Formation were also deposited during this period. ... 100 Figure 53: Drainage systems and source areas in a rotated fault block. A general illustration suggested by Faleide, Bjørlykke, and Gabrielsen (2010), which suits the deposition of the Agat Formation well. ... 102 Figure 54: Conceptual sketch of the Early Cretaceous Theory 1. The Selje High is still a prominent structural high during this period, and for this theory none of the faults interpreted in the study area is present. Submarine fans are deposited in the northern area and possibly at the western flank of Selje High. Slump deposits are deposited on the eastern plank of the Selje High. ... 103 Figure 55: Conceptual sketch of the Early Cretaceous alternative theory. The Selje High is still a prominent structural high during this period, and for this theory the faults have been reactivated. Submarine fans are deposited in the northern area and possibly at the western flank of Selje High. Slump deposits are deposited on the eastern plank of the Selje High. .. 104
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LIST OF TABLES
Table 1: Information about target, results, and lithology for the two wells 6204/10-1 and 6204/10-2R, which have drilled the basement. All information in the table is extracted from well reports for the two wells from NPD fact pages (NPD, 2019). ... 15 Table 2: List of wells in the seismic cube with additional information regarding check shots, side-tracks, target, and discoveries. Information is collected from the completion reports found on NPD fact pages (NPD, 2019). ... 20 Table 3: List of the seismic horizons interpreted in the study area and which wells have the well picks for the given horizons. ... 24 Table 4: Information about the three different spectral decomposition methods that are available and are used in this thesis. Information extracted and modified from a GeoTeric video tutorial online (GeoTeric, 2017b, 2017a). ... 26
1
INTRODUCTION
The first acquired exploration seismic on the Norwegian Continental shelf was in 1969, and the first license awarded in the Norwegian Sea in 1980 (Brekke, 2000). Since then, the understanding of the Norwegian margin has continuously improved; however, there are still some time periods and areas that need further investigation. For the Norwegian Sea margin, the initial focus was the late Palaeozoic and Mesozoic on the shallow shelf areas (Brekke &
Riis, 1987; Brekke, 2000). In later years, the deep-water sandstone deposits of Cretaceous and Palaeogene age in outer areas of the continental margin became of more interest (Brekke, 2000). These sandstone intervals are commonly interpreted as formed due to turbidite currents, and some as debris flows and seafloor currents transported from the mainland or from local structural highs (Brekke, 2000; Martinsen, Lien, & Jackson, 2005). Thus, the lateral extent of these reservoir rocks can be difficult to predict, and may show variation in reservoir quality (Martinsen et al., 2005).
The study area (Figure 1) is situated on the border between the northern North Sea and the southern Norwegian Sea at approximately 62°N. The main structural feature in the area is the basement high called the Selje Horst or Selje High. The study area is bounded by the Sogn Graben in the southwest, the Måløy Terrace in the south, the shallow platform area to the east and the Slørebotn Sub-basin to the northwest. All these structures are important in
understanding the distribution of the sandstones in the area.
There are few studies related to the Lower Cretaceous and the Upper Jurassic intervals in the study area. The Lower Cretaceous Agat Formation has proven to be a potential hydrocarbon bearing sandstone reservoir (Gulbrandsen & Nyborkken, 1991). The Agat Formation was first defined after the discovery of the Agat field in the northern North Sea and has later been identified in two wells in the study area in the southern Norwegian Sea (wells 6204/10-1 and 6204/11-2) (NPD, 2019). The distribution of this important sandstone formation in the Norwegian Sea is still poorly understood and the formation needs further investigation to better understand the thickness, depositional model and the lateral extent of which this
formation is distributed. In addition to the finding of the Agat formation in well 6204/10-1, an interval of unexpected conglomerates was discovered within the same group, the Cromer Knoll Group (NPD, 2019). The origin of these conglomerates is poorly understood but is initially thought to be eroded basement highs which are in close proximity to the area of deposition (NPD, 2019).
2
Only a few of the many wells at the Norwegian continental shelf have drilled pre-Devonian age, which has been defined as basement (Basset, 2003). Most of them are drilled on
structural highs where depth to the basement is noticeably less than in the basinal areas. The basement contains various rock types and has been exposed to different degrees of
metamorphose, fracturing and deformation (NPD, 2019). Furthermore, some of the wells show evidence of weathered rock materials, often conglomerates, creating a zone between the basement and the overlying sediments (NPD, 2019). A study on the basement can reveal if the basement is suitable as a reservoir, and how it influences the sedimentation of younger strata.
1.1
Objectives
The main purpose of this paper is to improve the understanding of the tectonostratigraphic evolution from Pre-Devonian Basement to Early Cretaceous in the area surrounding the Selje High. Special attention has been given to the Lower Cretaceous Agat sandstone Formation and its distribution and source area.
The following objectives are defined for the purpose of fulfilling the aim of this study:
• To establish the timing of major faults in the area and to define the syn-rift and post- rift stages to further understand the lateral distribution and origin of the sandstone formations in the area;
• To study the lateral distribution and origin of the coarse-grained sandstone intervals of the Agat Formation;
• To study the origin of unexpected conglomerates discovered in the Cretaceous Cromer Knoll Group in well 6204/10-1;
• To improve the paleogeographic understanding of the area; and
• To improve the understanding of the basement and its implications for exploration.
3
Figure 1: Study area (red square) and the structural elements of the northern North Sea and the southern Norwegian Sea. Gn H = Gnausen High, Go H = Gossa High, Gi H = Giske High, M H = Makrell Horst, and S H = Selje High. Modified from (Blystad et
al., 1995).
4 1.2
Previous work
Seismic expression of active rift basins
Prosser (1993) did a detailed study on depositional systems and their seismic expressions in active rift basins where four tectonic system tracts were defined; 1) rift initiation, 2) rift climax, 3) immediate post-rift, and 4) late post-rift. Figure 2 displays an idealized example of a basin with descriptions of the seismic characteristics one can possibly find in the given tectonic system tract.
Figure 2: An idealized section of an active rift basin with a description of seismic characteristics for the four tectonic systems tracts. 1) Rift initiation, 2) Rift climax, 3) Immediate post-rift, and 4) Late post-rift. (Prosser,
1993)
Færseth and Lien (2002) highlighted five elements that are commonly used to identify tectonic activity:
1) Onlap surfaces
2) Thickness variations across faults 3) Wedge-shaped sedimentary bodies 4) Variations in sedimentation rate 5) Influx of coarse clastics
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1.2.2 Rifting events in the Late Jurassic and Early Cretaceous and its influence on sedimentation
The tectonic evolution during Mesozoic times have been debated, and especially if the Jurassic rifting continued into the Cretaceous or not. The understanding of the tectonic
activity during Jurassic is highly relevant for the further understanding of the deposition of the Cretaceous interval.
Lundin and Dore (1997) suggested that the Early Cretaceous was tectonically active based on a series of observations. 1) Variation in thickness of the Lower Cretaceous succession, 2) wedge-shapes, and 3) offset of the base Cretaceous unconformity because of major faults.
These observations have also been made in the northern North Sea, which is better known than the Norwegian Sea, however, these have by other authors been considered as thermal subsidence and infilling of the topography caused by the Jurassic rifting event (Figure 3) (Bertram & Milton, 1988; Gabrielsen, Kyrkjebo, Faleide, Fjeldskaar, & Kjennerud, 2001;
Færseth & Lien, 2002).
Figure 3: Idealized section to illustrate Cretaceous infilling of half-grabens typical for the Cretaceous sedimentary succession in the Norwegian Sea. The Cretaceous infill has been interpreted as the post-rift infilling
of hanging wall sub-basins following Jurassic faulting and extensions (Færseth & Lien, 2002).
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Extensions in the Early Cretaceous have been considered part of the Jurassic rifting event in several papers (Blystad et al., 1995; McCann, Shannon, & Moore, 1995). However, Doré et al. (1999), proposed the rifting of Early Cretaceous to be a separate extensional event from the Jurassic, as the transition between the two events highlights the change of stress direction from E-W to NW-SE.
Færseth and Lien (2002) did a detailed interpretation of Cretaceous sedimentation in the Norwegian Sea with respect to the Jurassic rifting event until the Campanian rifting. The conclusion was that the deposition of the Cretaceous sediments was highly dependent on the tectonic configuration from the Jurassic rifting, but that the Early Cretaceous was a period of tectonic quiescence.
Figure 3 illustrates an idealized section of the Cretaceous infilling of the Jurassic rifts, where the Jurassic succession marks the syn-rift event and the Cretaceous succession is the post-rift event. This illustration substantiates the model described above for the northern North Sea.
1.2.3 Depositional model of the Agat Formation
The Agat field was found in the north-eastern part of the North Sea approximately 50 km off the Norwegian coastline and initiated the interest for the Agat Formation (Figure 1)
(Gulbrandsen & Nyborkken, 1991). This was the first field discovered offshore Norway in the Lower Cretaceous interval containing hydrocarbons. The Agat field is located between the mainland of Norway and the Sogn Graben. The sandstones of the Agat Formation is found mainly around the Måløy Terrace towards the Sogn Graben and is believed to be sourced from the narrow shelf area (Martinsen et al., 2005). The Agat Formation is of Aptian to Albian age (Gulbrandsen & Nyborkken, 1991; NPD, 2019).
After the discovery of the Agat field, the depositional environment of the Agat Formation has been debated, and several different models have been suggested.
Gulbrandsen (1987) suggested the first depositional model for the Agat Formation. The depositional model was based on the sands being sourced from multiple point sources on the shelf break in the east as submarine fans. Shanmugam et al. (1994) and Skibeli, Barnes, Straume, Syvertsen, and Shanmugam (1995) investigated several drilled wells in the Agat field looking at core data and proposing another depositional model comparing to
Gulbrandsen (1987). Based on the cores, they suggested a depositional model favoring slumps
7
and debris flows in an upper slope setting as the gravity processes causing the deposition of the Agat Formation.
In 1999, Nystuen published a new proposal for a model of the depositional environment of the Agat Formation, with more similar features to the Gulbrandsen (1987) model. Nystuen (1999) suggested that the dominated process for the Agat deposits was turbidity currents that were probably located within a channel system. Bugge, Tveiten, and Bäckström (2001) suggested a model also based on turbidity currents, however in their model the accommodation space was created in the slide scarps from slumping were sands accumulated as individual bodies. A general interpretation of the wells in the Agat field area shows turbidite deposition with occasional debris flows (Bugge et al., 2001).
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GEOLOGICAL SETTING
The study area is situated on the North-Atlantic continental margin which is a passive margin that has been exposed to multiple rift events before reaching its maximum when the
continental separation in the Norwegian Greenland Sea started in 53 Ma (Ziegler, 1975;
Blystad et al., 1995; Brekke, 2000; Vergara, Wreglesworth, Trayfoot, & Richardsen, 2001;
Osmundsen, Sommaruga, Skilbrei, & Olesen, 2002; Nasuti, Pascal, & Ebbing, 2012).
The study area is located on the boundary between the southern Norwegian Sea and the northern North Sea, where the NE – SW trending Møre-Trøndelag Fault Complex intersects with the structural trends of the northern North Sea (N – S and NNW – SSW) (Brekke, 2000).
The Møre-Trøndelag Fault Complex is marking the eastern boundary of the Møre Basin and is separating the deeper basin from the shallower minor basin areas (Figure 1). The highs and ridges in this complex are controlled by faults in the area and have NE – SW to ENE – WSW trends (Figure 1). (Brekke, 2000).
The Selje high, which is a horst structure, is the most prominent structure in the study area and is part of the Møre-Trøndelag Fault Complex. This structural high is a result of Jurassic- Early Cretaceous(?) faulting and is an uplifted footwall basement block. This block is part of a half-graben and has been rotated. The Selje High is located in the northern part of the Måløy Terrace (trending in a N – S direction), with the Sogn Graben (trending in a N – S direction) to the southwest and the Slørebotn sub-basin (trending in ENE – WSW direction) in the northwest (Figure 1) (Blystad et al., 1995). A regional seismic line in an E-W direction is presented in Figure 4, highlighting the geology in the study area.
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Figure 4: Regional seismic line crossing from West to East, from the Slørebotn Sub-basin through the Selje High to the Måløy Slope and the platform area. A) Uninterpreted seismic line. B) Interpreted seismic line. The interpretation in the west close to the platform area is uncertain.
NW SE A)
B)
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2.1 Tectonic setting
The Northeast Atlantic margin was tectonically active from Carboniferous to late Pliocene time (Brekke, 2000). Three main tectonic episodes formed the present tectonic architecture of the North Sea and the Norwegian Sea; 1) Permo-Triassic rifting; 2) Late Jurassic – Early Cretaceous(?) rifting; 3) Late Cretaceous rifting and subsequent drift and seafloor spreading (Brekke & Riis, 1987; Blystad et al., 1995; Brekke, 2000; Osmundsen et al., 2002; Martinsen et al., 2005).
The Iapetus Ocean closed during the pre-Late Devonian period and the Caledonian Orogeny (Brekke & Riis, 1987; Brekke, 2000). In the Devonian, a long event of rifting affected the eastern part of the North-Atlantic margin and a new period of several rifting events succeeded from Permian to Early Cretaceous with periods of tectonic quiescence in between (Blystad et al., 1995). These rifting events were a result of extensional tectonics (Brekke, 2000). In the Late Cretaceous to Tertiary the last rifting event occurred, however, this rifting event was a result of relative movements along plate boundaries in relevance to the beginning of the continental break up and sea-floor spreading of the North Atlantic margin separating the continents Eurasia and Greenland (Brekke & Riis, 1987; Blystad et al., 1995; Brekke, 2000).
The trends dominating the Northeast Atlantic margin are NE – SW, N – S and NW – SE faults, which also were subjected to reactivation during several rift phases (Osmundsen et al., 2002).
To further highlight the intervals of interest in this study, a more detailed explanation of the tectonic evolution of Jurassic and Cretaceous is given below.
2.1.1 Jurassic
In the Jurassic, a series of rifting events occurred with the most intense phase in the Middle to Late Jurassic (Vollset & Doré, 1984). These rifts were influenced by the structural framework created in the Triassic.
In the Late Jurassic significant changes in the geology followed which resulted in the development of new landscapes (Nøttvedt & Johannessen, 2008). A series of rifting events influenced the North Sea and resulted in the creation of the Viking and Central Graben (Ziegler, 1975). Fault blocks were rotated due to normal faulting along the Viking Graben (Faleide et al., 2010). The first period of rifting began in the North Sea in the south and moved all the way to the Barents Sea in the north (Vollset & Doré, 1984; Nøttvedt &
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Johannessen, 2008). The rifting in Jurassic has different interpretations, however, Faleide et al. (2010) suggested that the rifting period lasted only to the end of Jurassic for the northern North Sea, while the rifting in the Norwegian Sea ended in the Early Cretaceous. The Late Jurassic rifting was different from the previous rifting period of Permian to Triassic, as the Late Jurassic rift thinned the crust severely and the whole rift submerged into the sea. Most of the faults of Jurassic age were terminated before reaching the end of Jurassic, but a few faults are evident in younger strata (Faleide et al., 2010).
2.1.2 Cretaceous
Faleide et al. (2010) suggested that the rifting from Jurassic died out in the Cretaceous, and that the entire Cretaceous succession is post-rift strata. The ending of rifting resulted in the crust cooling and subsidence of the Cretaceous basins occurred (Faleide et al., 2010).
Furthermore, Færseth and Lien (2002) also explained that the Cretaceous period was dominated by tectonic quiescence and thermal subsidence.
However, several other authors claimed that the Early Cretaceous showed evidences of active rifting. Lundin and Dore (1997) explained the tectonic of Early Cretaceous in this sense. The North Sea rift from Jurassic age got extinct in the Kimmeridgian and subsidence occurred as explained by Færseth and Lien (2002) and Faleide et al. (2010). However, sea floor spreading in the Atlantic were moving northwards to the Møre Basin in the Early Aptian leading to NW- SE extensions, which were different from the Late Jurassic trends. Thus, the conclusion was that the Late Jurassic and the Early Cretaceous were two separate events, but the time in between the events was not necessarily large (Lundin & Dore, 1997).
A regional geological line highlighting the Selje High, Slørebotn Sub-basin and Måløy Terrace is presented in Figure 5.
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Figure 5: Regional line showing from the NW to the SSE from the Slørebotn sub-basin, Måløy Terrace and Selje High. This regional line is along the same line as interpreted in Figure 4. Modified by (Brekke, 2000).
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2.2 Stratigraphy
In the most southern parts of the Norwegian Sea, around 62-63°N, the lithostratigraphy is considered more like that of the northern North Sea than the Norwegian Sea, and the lithostratigraphic names in this area are therefore the same as in the North Sea (Figure 6) (Vergara et al., 2001). The deposition in the North Sea during Early Cretaceous has been interpreted as heavily dependent on the basin topography created by rifting during the Late Jurassic (Martinsen et al., 2005). The accommodation space, source area, and transport direction for the eastern margin were controlled by the structural highs, terraces, and the slopes (Martinsen et al., 2005).
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Figure 6: Lithostratigraphic chart of the Northern North Sea from the Tampen Spur and Horda Platform. The lithostratigraphy in the study area is believed to be similar. Modified from (NPD, 2014).
15 2.2.1 Pre-Devonian
A small number of the wells drilled in the Norwegian continental margin have penetrated basement rocks of Pre-Devonian age, thus this interval is fairly documented and has less certainty than overlying successions (Basset, 2003). The basement consist of several different rocks such as igneous rocks, metamorphic rocks, and metasedimentary rocks (Basset, 2003).
The Pre-Devonian sediments are highly fractured and deformed due to metamorphism and are forming the basement of the northern North Sea and southern Norwegian Sea (Gee & Sturt, 1985; Pickering, Bassett, & Siveter, 1988; Basset, 2003).
The basement is encountered in two of the wells in the study area, wells 6204/10-1 and 6204/10-2R. It is therefore relevant to look at the well report for the two wells to compare the lithology found in the basement (Table 1); however, none of the wells have cores from the basement interval.
Table 1: Information about target, results, and lithology for the two wells 6204/10-1 and 6204/10-2R, which have drilled the basement. All information in the table is extracted from well reports for the two wells from NPD
fact pages (NPD, 2019).
Well
Target Results Lithology of basement Lithology of
conglomerates above 6204/10-1 J-prospect
(Turonian).
Jurassic Lead.
Stratigraphy in Tertiary, Turonian and Cenomanian levels.
Proved geological model of the J- prospect.
Unexpected conglomerates in the lower parts of the well.
The lowermost part penetrated in the well consists of conglomeratic sediments with the same composition as the
conglomerates described to the right. Uncertain age of sediments, either Triassic or Basement.
Conglomerate comprises of rock fragments (gneiss, quartzite, mica schist) locally quartz fragments, quartz/feldspar, different kind of mica, pyrite, and traces of rose quartz.
6204/10-2R Jurassic L prospect.
Turonian/Coniacian Q-prospect.
Secondary target:
Fractured basement.
Jurassic section missing. No hydrocarbons encountered in the basement. Thin Lower Cretaceous sandstone stringer with gas.
Cuttings show rock fragments of metamorphic rocks, gneiss, mica, schist, quartz, feldspar, muscovite and biotite. Traces of white opaque crystalline calcite constitute vein or fracture fill.
Thin conglomeratic sandstone on top of the basement which grades into sandy conglomerate.
The conglomerate is sandy with rock fragments of the metamorphic basement.
16 2.2.2 Pre-Jurassic
During Permian, the climate became arid and dry, and the basins were filled with continental sediments as alluvial fans and aeolian sand dunes of the Rotliegend formation (Larsen, Olaussen, Sundvoll, & Heeremans, 2007; Faleide et al., 2010).
In the Triassic the climate was dry, and the period was dominated by sands, gravels and mud deposits coming from the elevated areas around creating alluvial plains (Ramberg, Solli, Nordgulen, Binns, & Grogan, 2008). Fine-grained material were deposited in the basins, while coarse-grained deposits were found in the marginal areas (Vollset & Doré, 1984).
During the late Triassic, a dramatic change in climate led to seawater flooding the alluvial plains and ending the continental deposition (Vollset & Doré, 1984; Ramberg et al., 2008).
2.2.3 Jurassic
The continental areas became covered in shallow seas due to a transgression in the Early Jurassic and marine shales were deposited over large areas (Vollset & Doré, 1984; Faleide et al., 2010). During Middle Jurassic, volcanic activity occurred in the central part of the North Sea causing dome structures. These structures worked as source areas for deltaic deposits, and the deposition of the Brent Group occurred in the northern North Sea (Vollset & Doré, 1984;
Faleide et al., 2010).
In the Late Jurassic, the Viking Graben was exposed to normal faulting causing rotation of basement blocks, resulting in the blocks being tilted and the upper part of the footwall becoming exposed to erosion (Vollset & Doré, 1984; Faleide et al., 2010). This led to the removal of Lower to Middle Jurassic and in some areas even Upper Triassic strata (Faleide et al., 2010). A period of transgression in the Late Jurassic covered the northern North Sea with seawater leading to the deposition of the clay sediments of the Heather Formation; however, some coarse-grained sediments from turbidites and deltas were also deposited off from the margins (Sognefjord Formation and Heather Sandstone Formation) (Vollset & Doré, 1984;
Faleide et al., 2010).
2.2.4 Cretaceous
The Early Cretaceous is recognized by a transgression interchanging with minor regressive events resulting in mainly deposits of shales (Isaksen & Tonstad, 1989; Brekke & Olaussen, 2008; Faleide et al., 2010). The change from Jurassic to Cretaceous is marked by a major
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unconformity called the Base Cretaceous Unconformity (BCU) (Faleide et al., 2010). Several formations have been identified in the Early Cretaceous, such as the shaly Åsgard, which were deposited during quiet conditions and a transgression in Valanginian to late Barremian (Faleide et al., 2010). Movements along the North Sea rifts during Mid-late Aptian together with the occurrence of a regression caused changes in the lithology in the northern North Sea.
As a result, the previously calcareous rich claystones in the basinal areas changed to more organic-rich claystones creating a new formation called the Sola Formation (Isaksen &
Tonstad, 1989; Faleide et al., 2010). In addition, the sandstone rich sediments of the Agat Formation and the Ran sandstone Member were deposited during this period. These sandstone formations were a result of erosion along the flanks of structural highs and were deposited as gravity flows in some parts of the northern North Sea and the southern Norwegian Sea, like the Måløy Terrace and around the Selje High (Isaksen & Tonstad, 1989).
As the regression changed into transgression during Albian, the deposition of the Agat and Ran sandstones were still active as some of the higher parts of the structural highs were still exposed to erosion, while the overall area was covered by the sea (Isaksen & Tonstad, 1989).
During this period, a new formation called the Rødby Formation consisting of fine-grained calcareous sediments evolved (Isaksen & Tonstad, 1989; Faleide et al., 2010).
In the Late Cretaceous, the northern North Sea was tectonically relatively quiet and the transgression reached its maximum, resulting in low clastic sedimentation and the area was now dominated by the marine shaly carbonate Shetland Group (Isaksen & Tonstad, 1989;
Faleide et al., 2010).
Cores from the Åsgard Formation in well 6204/10-2R are extracted from NPD and displayed in Figure 7. The cores show the description of dark grey to grey shaly sediments as explained above.
18 2.2.4.1 Agat Formation
The deposition of the Agat Formation occurred during the late Early Cretaceous on the Måløy Slope, illustrated in the chronostratigraphic chart in Figure 8 (Isaksen & Tonstad, 1989;
Martinsen et al., 2005). The formation has a high content of coarse-grained sandstones that are the result of regressive events causing mass-flows along the slope (Bugge et al., 2001;
Martinsen et al., 2005). The sandstones are therefore interbedded with shale creating sand beds with a thickness of 10-30 cm (Martinsen et al., 2005). Locally the sandstone beds can become tens of meters in thickness (Bugge et al., 2001; Martinsen et al., 2005).
The deposition of the Agat Formation is from the shelf in the east to the basin in the west (Gulbrandsen & Nyborkken, 1991; Bugge et al., 2001; Martinsen et al., 2005). Several different elements are proven, such as slumps, channels and sheet bodies, which might imply transformation in the slope angle. (Gulbrandsen & Nyborkken, 1991; Skibeli et al., 1995;
Martinsen et al., 2005).
Figure 7: Cores from well 6204/10-2R showing the Åsgard Formation from 1951m to 1961m. The cores show dark fine-grained sediments. Collected from NPD fact pages (NPD, 2019).
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Figure 8: Chronostratigraphic chart for the Cretaceous from the Sogn Graben, Måløy Terrace, and Selje High.
(Modified from (Vergara, Brunstad, Nordlie, Chranock, & Gradstein, 2006)
2.2.5 Post-Cretaceous
The sedimentation and deposition during the Cenozoic period were greatly affected by the uplift of surrounding clastic source areas (Faleide et al., 2010). The North Sea was not affected much by the spreading of the Norwegian Sea, and the area was relatively stable in Paleogene and Neogene except for some periods of uplift (Martinsen & Nøttvedt, 2007).
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DATA AND METHODOLOGY
3.1 Data
A 3D seismic cube covering an area of 518 km2 were available in the dataset. In addition to the 3D seismic, a large number of regional 2D seismic lines were available, however, these were used only for regional interpretation (Figure 9). The vertical resolution of cube ST9202 varies with depth, but the resolution for the Agat Formation in the area close to well 6204/10- 1 is 42m, while in well 6204/11-2 the resolution in the Agat Formation is 48m.
The dataset contained six wells, where two are side-tracks of well 6204/10-2 (Table 2). The wells included conventional well logs such as gamma ray (GR), density (RHOB), sonic (AI), resistivity (Res), spontaneous potential (SP), and others. Three wells have check shots used for creating a time-depth table. Further well information and core descriptions were available at the fact pages of the Norwegian Petroleum Directorate (NPD). None of the wells in the study area has cores from the Agat Formation, the conglomerates or the Basement.
Table 2: List of wells in the seismic cube with additional information regarding check shots, side-tracks, target, and discoveries. Information is collected from the completion reports found on NPD fact pages (NPD, 2019).
Wells Total depth (MD)
Check shot
Side- track
Target Results
6204/10-1 2709 m X J-prospect (Turonian). Jurassic Lead.
Improve stratigraphic understanding in Tertiary, Turonian and Cenomanian levels.
Dry
6204/10-2 1145 m Jurassic L-prospect. Coniacian/Turonian Q- prospect. Secondary target: Fractured
basement
Dry No discovery in
the basement 6204/10-2
A
2290 m X Prove economic hydrocarbons and down-dip
reservoir thickening of the gas-filled sand penetrated in well 6204/10-2R.
Dry
6204/10-2 R
2095 m X X Jurassic L-prospect. Coniacian/Turonian Q- prospect.
Secondary target: fractured basement.
Gas shows No discovery in
the basement 6204/11-1 2966 m Middle Jurassic Brent Group Equivalent in
the E-prospect.
Sandstones of Turonian age in the I-prospect.
Gas discovery GWC: 2792.5m
TVD RKB 6204/11-2 2920 m X I prospect (Coniacian/Turonian).
O-prospect (Albian/Aptian)
Oil shows
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Figure 9: An overview of the dataset from the study area. Blue lines are 2D seismic lines, while the black box show the seismic 3D cube. Black circles are the wells. The Norwegian mainland are located to the east.
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3.2 Methodology
3.2.1 Well correlation
Well correlation was executed based on the well tops information provided by NPD (2019).
The well correlation was done with the intention of getting a better understanding of the wells in the area before starting the seismic interpretation. For that purpose, a structural well
correlation with a focus on the Early Cretaceous and Jurassic interval to highlight structural features was done. The different formations were correlated across the area in all wells from SW to NE and the well correlation started at the top of Shetland Group (Early Cretaceous).
3.2.2 Seismic interpretation
Firstly, it is important to establish the polarity of the seismic data. To identify this the seabed was used where the acoustic impedance is going from low to high. If the reflector is red and red represents negative amplitude, then the polarity is considered reverse. In the opposite case, the polarity would be normal. Figure 10 displays the seabed in seismic cube ST9202, and the polarity is interpreted to be reversed for this cube.
A seismic well-tie was executed in the three wells that had check shots, wells 6204/10-1, 6204/10-2R, and 6204/11-2. The extracted wavelet was not a perfect fit, but by adjusting a Ricker wavelet to cover the dominant frequency it fitted well. For wells 6204/10-1 and 6204/11-2 a Ricker wavelet with a frequency of 20 Hz and phase rotation of 0 degrees was used. The synthetic seismogram is fitted specifically to the interval of interest, the Cromer Knoll Group, and Agat Formation. When displaying the seismic well tie in the seismic it fits well with the reflectors, and the horizons can easily be identified for mapping (Figure 11).
The result from the synthetic well-tie in wells 6204/10-1 and 6204/11-1 are displayed in Figure 12 and Figure 13.
Figure 10: A close-up of the seabed highlighting the
polarity of the seismic cube.
Figure 11: Seismic displayed with the synthetic seismogram to highlight the good correlation between the
synthetic and the seismic.
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Figure 12: Seismic well-tie for well 6204/10-1. The following well logs displayed are: GR, sonic, density, acoustic impedance, and reflection coefficient. The top of present formations are marked.
Figure 13: Seismic well-tie for well 6204/11-2. The following well logs displayed are: GR, sonic, density, acoustic impedance, and reflection coefficient. The top of present formations are marked.
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Four key horizons were identified based on answering the objectives; Top Basement, Base Cretaceous Unconformity, Base Agat Formation, and Top Agat Formation. The BCU in the study area is defined as the base of the Cretaceous Cromer Knoll Gp. All the key horizons have only been identified in some of the wells, which made the interpretation slightly difficult in certain areas. An overview of the different horizons and in which wells they are identified are given in Table 3. Well 6204/10-2 did not contain any of the chosen horizons.
Table 3: List of the seismic horizons interpreted in the study area and which wells have the well picks for the given horizons.
All the main faults in the study area were interpreted and displayed on the structural map created from the interpreted horizons. It is worth mentioning that only the major faults from the Basement to Lower Cretaceous have been mapped in this thesis.
Furthermore, all the main faults in the study area were interpreted with respect to syn-rift and post-rift, based on the elements shown in Figure 2. The faults were separated into fault families containing similar age, strike, and location.
Lastly, the integration of the data contained in this study was used to create conceptual sketches of the paleogeography from the Late Jurassic to Base Cretaceous interval.
Seismic horizon 6204/10-1 6204/10-2 6204/10-2R 6204/10-2A 6204/11-1 6204/11-2
Top Basement X X
Top Agat Fm X X
Base Agat Fm X X
BCU X X ? X X
25 3.2.3 Spectral decomposition
For further investigation of the interval of interest, spectral decomposition was done in GeoTeric. The Agat Formation is a thin sandstone unit (106m in well 6204/10-1 and 30m in well 6204/11-2), which is difficult to distinguish in some areas of the seismic due to the vertical seismic resolution (40-48m).
Theory
Spectral decomposition is a method used to highlight seismic features and attributes and can help distinguish the top and base of a layer beneath the seismic resolution (Partyka, Gridley,
& Lopez, 1999). By using spectral decomposition, it increases the accuracy and effectiveness of seismic interpretation by highlighting geological features such as channels, faults and bed thickness that otherwise could be buried in seismic broadband data (Li, Qi, Marfurt, & Stark, 2015; Chopra & Marfurt, 2016).
Iso-proportional slicing is a method to slice up the 3D seismic volume (Posamentier, Davies, Cartwright, & Wood, 2007). Well-mapped horizons can be used as a base for the slicing in between an interval of interest. The interval between the two horizons are divided into a number of chosen slices, thus revealing better the stratigraphic features of the interval
(Posamentier et al., 2007; Cader, 2017). Lastly, the color blending can be applied to the slices.
Color blending is a powerful tool, where different colors are assigned to different volumes (Moore & Smith, 2017). Thus, multiple spectral components are blended into one image.
White color indicates that all the three volumes are high, however, if one of the volumes dominate, the assigned color for that volume will be evident (Li et al., 2015). Black indicates that all three volumes are present at more or less the same amount (Li et al., 2015; Moore &
Smith, 2017).
The different spectral decomposition options are listed in Table 4, highlighting the methods and use.
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Table 4: Information about the three different spectral decomposition methods that are available and are used in this thesis. Information extracted and modified from a GeoTeric video tutorial online (GeoTeric, 2017b, 2017a).
Methodology
The software used to do spectral decomposition is GeoTeric. The main attributes used from the software were frequency decomposition, HD frequency decomposition and iso-
proportional slicing all with color blending (GeoTeric, 2017b).
All four different decomposition options were tested on the seismic cube in GeoTeric. After evaluating the results of the decomposition, only two composition methods were used to describe the results; Constant bandwidth for both Basement and Agat Formation and HD frequency decomposition for only the Basement. In the HD frequency decomposition, the following frequencies were used: 15Hz – 30Hz – 45Hz for the different volumes for the Basement. For the constant bandwidth, another set of frequencies were used for the Basement and Agat Formation: 17Hz – 24Hz – 31Hz and 17Hz – 30Hz – 42Hz respectively. These volumes were assigned to different colors to create the color blend, where the lowest
frequencies are red, medium are green and highest are blue. The resultant color blends were used on the surface of both the Basement and the Agat Formation to highlight stratigraphic features on the surface.
In addition, iso-proportional slicing between the top and base of the Agat Formation with 3 slices in between were executed. The constant bandwidth color blend for the Agat Formation was assigned to these slices and used to evaluate the changes in the formation.
Decomposition options Methods Use
Standard Frequency Decomposition
Constant Bandwidth Similar to Fourier Transform
Estimation of bed thickness and thin bed interference.
Constant Q (Exponential and Uniform)
Similar to a Continuous Wavelet Transform
Regional studies and reconnaissance.
High Definition Frequency Decomposition
HD Frequency Decomposition
Modified matching pursuit
Reservoir-scale analysis and understanding depositional
layers.
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OBSERVATIONS AND RESULTS
4.1 Structural well correlation
Observations
The structural well correlation (Figure 14) is concentrated mainly in the interval of interest from the Basement until the Upper Cretaceous. All the well tops used in this correlation are collected from the Norwegian Petroleum Directorates facts pages (NPD, 2019).
The section goes from the Selje High in the south to the Slørebotn Sub-basin in the north (map in Figure 14). The basement is only encountered in two of the wells, 6204/10-1 and 6204/10-2R. The difference in elevation is striking, with a much thicker interval of the basement for the well drilled in the Selje High.
The Jurassic section is only present in two of the five wells in the correlation (6204/11-1 and 6204/11-2), and only the Upper Jurassic is penetrated (Heather and Sognefjord formations).
Whereas in the Jurassic the thickest interval is located in well 6204/11-1, in the Cretaceous the thickest interval is located in well 6204/11-2. The Jurassic interval in well 6204/11-1 has a thickness of 314m, while in well 6204/11-2 the thickness is at least 92m, however, this well is not drilled deeper. The Lower and Middle Jurassic is missing, and the Upper Jurassic in well 6204/11-1 overlays a thin Triassic interval while well 6204/11-2 is terminated in the Jurassic Sognefjord Formation. The BCU is defined at the base of the Cromer Knoll Group, which is located at 2602m in well 6204/11-1 and 2769m in well 6204/11-2.
The Lower Cretaceous Cromer Knoll Group consists of a thin interval varying from 138m in the elevated basement to 0-205m in the lower areas; however, the thickness variation between wells is significantly less compared to the Upper Cretaceous. Still, there are some small variations in thickness and elevation.
Furthermore, the wells drilled on the Selje High, have the Lower Cretaceous interval drilled at 1935m and 2233m, while two of the remaining wells (6204-10/1 and 6204/11-2) have drilled the same group at a much deeper level at 2465m and 2659m. Moreover, the well 6204/11-1 did not encounter the Lower Cretaceous interval. Thus, the Lower Cretaceous interval have varying formations present in the wells. The Rødby Formation and the Agat Formation is only present in wells 6204/10-1 and 6204/11-2. It is worth mentioning that these two wells are located in the structural lows in the study area. The two formations, Rødby and Agat, show a much thicker interval in well 6204/10-1 located in close proximity to the Selje High, with
28
99m and 106m, respectively, compared to 31m and 30m in well 6204/11-2. Thus, the Agat Formation is not observed in any of the wells drilled in the structural highs (wells 6204/10- 2A, 6204/10-2R, and 6204/11-1).
The Åsgard Formation underlies the Agat Formation in well 6204/11-2 as a thin unit of 39m, however, this formation is not present below the Agat Formation in well 6204/10-1 which non-conformably lies on top of the basement. In addition, the Åsgard Formation is penetrated in wells 6204/10-2A and 6204/10-2R at the Selje High with a thickness of 138m in the latter.
Moreover, this is the only formation encountered of Lower Cretaceous age in the Selje High.
The Shetland Group marks the top of the correlation, with either Jorsalfare Formation or the Kyrre Formation depending on the well. The Upper Cretaceous section consists of a very thick interval varying from approximately 1250m to 1400m in the three wells 6204/10-1, 6204/11-1 and 6204/11-2, while the other two wells have considerably thinner sections varying from approximately 650m to 950m of Upper Cretaceous succession.
Lastly, the cores in Figure 7 of the Åsgard Formation are relevant to mention here as it might give indications of the depositional environment during the Early Cretaceous. The cores are from well 6204/10-2R drilled in the Selje High. The main observation is the dark gray color which becomes slightly lighter towards the end at 1961m. Moreover, the cores look overall fine-grained and well sorted, with a small interval of coarser material at the bottom of 1656- 1657.
Interpretation
The most evident observation is the displacement between the wells drilled on the Selje High and the surrounding areas. Before doing any seismic interpretation, this looks like a major normal fault displacing the wells 6204/10-2A and 6204/10-2R up compared to the other wells, which could also explain why the sediments of the Jurassic interval are eroded away or not deposited at the structural high. However, well 6204/10-1 has also been exposed to erosion or non-deposition of the Upper Jurassic interval. These wells are located at the Selje High and the Måløy Slope (6204/10-1), while wells 6204/11-1 and 6204/11-2 are located in the deeper Slørebotn Sub-basin.
In addition to the possible major fault in the south, the elevation difference between the Jurassic intervals in wells 6204/11-1 and 6204/11-2 might indicate another normal fault in the correlation, however, this would be a minor fault. Moreover, the change in thickness from
29
Jurassic to Cretaceous across faults further confirm the possible fault activity. The presence of the Heather Formation in Late Jurassic suggests deep marine conditions associated with the transgression during this period (Faleide et al., 2010), while the Sognefjord Formation and the Heather Sandstone (SS) Formation are indicating infill of clastic sediments from the platform area, likely during minor regressive events as explained in the geological setting (Bugge et al., 2001).
Furthermore, the thickness variations and absence of the Upper Jurassic and the Agat Formation in the structural highs and the shift in the thickness of the Upper Jurassic and Lower Cretaceous between wells 6204/11-1 and 6204/11-2 further indicate fault activity during the deposition of the Late Jurassic and the Early Cretaceous.
The presence of the Åsgard Formation in some of the structural highs suggests that they were partially flooded in the Early Cretaceous before the deposition of the Agat Formation in the structural lows. However, it is interesting that the Åsgard Formation is only present in the Selje High and well 6204/11-2, which is good indications of a period of active rifting and erosion after the deposition of the Åsgard Formation. In addition, the cores displayed in Figure 7 showing fine-grained dark sediments are good indications that the Åsgard Formation was deposited during a transgression, which corresponds well with the Early Cretaceous transgression suggested by Faleide et al. (2010).
However, this hypothesis needs to be further investigated with seismic data.
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Figure 14: Structural well correlation between all wells in the study area going from southwest to northeast (yellow line in map), through the Selje High, Måløy Slope, and Slørebotn Sub-basin. The correlation is from Basement until the top of Upper Cretaceous.
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4.2 Seismic interpretation of the main intervals
In this subchapter, the observations based on the three main units; Basement, Pre-BCU, and Agat Formation will be presented separated into faulting, mapping, seismic character, and spectral decomposition. In addition, some basic interpretations are presented for further discussion in Chapter 5.
4.2.1 Basement
In this part, the observations and interpretations based on the basement are presented. These are crucial for understanding the structural elements in the study area but might also contain important information about the depositional history and help with the understanding of the overlying sediments.
4.2.1.1 Faulting
Three fault families were identified in the study area during the structural interpretation and these are differentiated based on orientation, timing, and location. Figure 15 shows all the eleven major faults mapped in the study area. Faults are marked with the given fault family in a structural map of the basement. A further detailed description of the different families is provided below.
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Figure 15: Structural map of the basement with all the faults divided into three fault families. Fault family 1(FF1) in yellow, Fault family 2 (FF2) in green and Fault family 3 (FF3) in blue.
